The present invention relates in general to telecommunication techniques. More particularly, the invention provides a method and system for automatic chromatic dispersion compensation. Merely by way of example, the invention is described as it applies to optical networks, but it should be recognized that the invention has a broader range of applicability.
Telecommunication techniques have progressed through the years. As merely an example, optical networks have been used for conventional telecommunications in voice and other applications. The optical networks can transmit multiple signals of different capacities. For example, the optical networks terminate signals, multiplex signals from a lower speed to a higher speed, switch signals, and transport signals in the networks according to certain definitions.
In optical communications, an optical signal may transmit a long distance, such as hundreds or even thousands of kilometers, in single mode optical fiber links. An important property of optical fibers is chromatic dispersion, that is different spectral components of the signal travel at different speed in the optical fibers. The chromatic dispersion may broaden the signal pulses and limit the transmission distance. For example, a single mode fiber (SMF) has a chromatic dispersion of 17 ps/(nm×km) at a signal wavelength of 1550 nm. If the spectral width of the signal is 0.1 nm, the signal pulses would become 170 ps wider after a transmission distance of 100 km. For high-speed transmissions at over one gigabit per second, the bit periods are only a few hundred picoseconds, or even a few tens picoseconds; thus such broadening can significantly degrade the detectability of the signal.
The pulse broadening is related both to the spectral width of the optical signal and dispersion of the optical fiber. To improve the dispersion-limited transmission distance, it is desirable to narrow the spectral width of the optical signal. Some transmitters use directly modulated semiconductor diode lasers (DMLs) to generate an optical signal. DMLs usually introduce additional frequency modulations to the optical signal, such as a chirp on top of the intensity modulated signal, and broaden the signal spectrum. The broadening of the signal spectrum may in turn reduce the dispersion-limited transmission distance. In contrast, other transmitters use continuous wave (CW) semiconductor diode lasers and low-chirp external modulators, which introduce less spectral broadening.
The spectral width of a chirp-free optical signal is given by the Fourier transform limit. The spectral width is about equal to the inverse of the minimum pulse duration, or equal to the data rate. For example, a non-return-to-zero (NRZ) binary signal has a data rate of 10 Gbps and the minimum pulse duration of 100 ps. The spectral width is about 10 GHz or 0.08 nm. If the signal transmits 70 km in a single mode fiber (SMF) with a cumulative dispersion of 1200 ps/nm, the signal pulses would be broadened by about 100 ps. This broadening is approximately equal to the bit period. Thus the dispersion-limited transmission distance of a chirp-free 10-Gbp NRZ optical signal is about 70 km in SMF.
To transmit beyond the dispersion-limited transmission distance, the dispersion compensation is usually required. A conventional method for compensating chromatic dispersion in optical fibers uses dispersion compensating fiber (DCF), which exhibits a negative chromatic dispersion. For example, segments of DCF are inserted in transmission lines between individual fiber spans at nodes where other transmission procedures are performed. These transmission procedures may include optical amplification and optical channel add/drop. The negative dispersion value of DCF at each node is selected so that the cumulative total dispersion at the node is close to zero. Additionally, at the end of the transmission, just before the receiver, the cumulative dispersion should be at an optimal value where the distortion of the signal is minimal. For example, at the limit of linear transmission of a chirp-free signal, this optimal value is equal to zero. When other factors are accounted for, the optimal value may shift away from zero.
Due to variations in fiber routes, the actual cumulative dispersion for any given compensated transmission line can vary by a large amount. Deviations from the optimal value of cumulative dispersion may cause penalties to the receiver performance. The tolerance of a transmission system to such deviations is called dispersion compensation tolerance, or dispersion tolerance. The optimal value for the cumulative dispersion is referred to as the center of the dispersion tolerance window.
Many factors may cause deviations from the optimal cumulative dispersion. Among them are the length of a fiber span, the dispersion of a transmission fiber, and the dispersion of a dispersion compensating fiber. For example, unforeseen fiber cuts and repairs may change the value of the cumulative dispersion. A change of 20 km in the length of a single mode fiber may shift the cumulative dispersion by 340 ps/nm. Additionally, the fiber dispersion is affected by temperature and aging.
In dense wavelength division multiplexing (DWDM) transmissions, the chromatic dispersion and its compensation is complicated. Fiber dispersion is usually wavelength dependent, and the dispersion slope is usually about 0.05-0.09 ps/(nm2×km). For DWDM transmissions, dispersion compensating fibers (DCFs) should usually have negative dispersion slopes. Variations in dispersion slopes are often limited to 10%, which means the dispersion slopes in transmission lines can only be compensated up to 90%. For a transmission line of 1000 km, the cumulative dispersion variation across a DWDM transmission window, such as from 1530 nm to 1562 nm in C-band, could vary by0.09 ps/(nm2×km)×32 nm×1000 km×10%=288 ps/nm  (Equation 1)
In other example, certain low-cost DCFs can provide only 60% slope compensation; hence the cumulative dispersion variation increases to 1152 ps/nm.
In order to improve dispersion compensation, adjustable optical dispersion compensators have been proposed. For example, an adjustable optical dispersion compensator is similar to a disperse compensation fiber with a varying length. The length variation can either continuously or in steps adjusts the value of cumulative dispersion. When used, the value of cumulative dispersion can be adjusted to minimize the distortion on the received signals. If an adjustable optical dispersion compensator has a sufficient range, the compensator can be adjusted to obtain a total cumulative dispersion to optimize the receiver performance. The range of the adjustable dispersion varies with the underlying mechanism of the dispersion compensator. Usually, adjustable optical dispersion compensators with large adjustment ranges are bulky and costly.
As another example, electronic dispersion compensators can provide adjustable dispersion compensation. For example, an electronic dispersion compensator restores, after the optical-to-electrical conversion, the received signal distorted by the dispersion. After the signal restoration, the dispersion tolerance window becomes wider, but the center of the dispersion tolerance window remains the same. Since the dispersion occurs in the optical domain but the compensation is performed in the electronic domain, the compensation is usually very limited. For example, electronic dispersion compensators can increase the width of the dispersion tolerance window by about 50%.
Hence it is highly desirable to improve techniques for compensating chromatic dispersion in optical networks.